The internal power unit may have a regulation feature that keeps B+ constant over a range of input, the heaters likely would not, and higher than normal heater voltage giving hotter than normal cathode, increasing electron emission, would be somewhat equivalent to running higher B+. This would definitely affect the operating points, it probably wouldn't increase peak output level as increasing B+ may.pio,
i can't answer if 15v would be too much for the pre to handle, but i'd measure the power supply voltage before i'd try it if i was feeling adventurous
the pre is really inexpensive so maybe the heaters would burn up? if you have some extra tubes, or the original inexpensive tubes & you think you'd like to experiment with tube rolling, i'd say go for it as probably only the tubes would be damaged?
it it doesn't immediately fail, maybe the life of the tubes would be shortened, but it might sound better?
would the operating point of the tube change with a higher voltage to the plate? maybe someone with a handle on tube amp design can chime in
Otherwise, if the B+ were to also increase, then the effect with the hotter cathodes on the operating points would be greater still. Observational effects vs measurements of this abnormal state might be useful for curiosity sake, intentional design is always better.
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